Tissues and Early Embryology

Introduction to Tissues

The human body is organized into levels of increasing complexity, from molecules and cells to tissues, organs, and organ systems. Tissues are groups of similar cells that perform specific functions. There are four primary tissue types in the human body: epithelial, connective, muscle, and neural tissues.

• Epithelial Tissue: Covers exposed surfaces and lines internal cavities and passageways.

• Connective Tissue: Fills internal spaces, provides structural support, and stores energy.

• Muscle Tissue: Contracts to produce active movement.

• Neural Tissue: Conducts electrical impulses and carries information.

Epithelial Tissue

Epithelial tissue consists of sheets of closely packed cells that cover body surfaces and line internal organs and cavities. This tissue type serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Definition: An epithelium is a sheet of cells that covers exposed surfaces and lines internal cavities and passageways.

• Main Functions:

  • Physical protection: Shields underlying tissues from mechanical and chemical stress.

  • Control permeability: Regulates the movement of substances into and out of the body or organs.

  • Provide sensation: Contains sensory nerve endings to detect changes in the environment.

  • Produce secretions: Forms glands that secrete substances such as enzymes, hormones, and mucus.

• Specializations of Epithelial Cells:

  • Microvilli: Finger-like projections that increase surface area for absorption (e.g., in the intestines).

  • Stereocilia: Long, non-motile microvilli found in the male reproductive tract and inner ear.

  • Ciliated epithelium: Contains cilia that move substances across the epithelial surface (e.g., respiratory tract).

Summary Table: Tissue Types and Functions

Key Terms

• Epithelium: A sheet of cells covering body surfaces or lining cavities.

• Microvilli: Small projections that increase cell surface area.

• Cilia: Hair-like structures that move substances over epithelial surfaces.

• Gland: An organ that produces and releases substances for use in the body or for elimination.

Example: Epithelial Tissue in the Respiratory Tract

The lining of the trachea is composed of ciliated epithelial cells. The cilia move mucus and trapped particles upward toward the throat, helping to keep the airway clear.

Additional info: Later sections of this chapter will cover the classification of epithelial tissues (simple vs. stratified, squamous vs. cuboidal vs. columnar), glandular epithelium, and the embryological origins of tissues.